Predicting electret performance by measuring level of extractable hydrocarbons

ABSTRACT

A method of predicting the performance of electret filter web involves measuring the level of extractable hydrocarbon material in the web. Electret filters that have low extractable hydrocarbon levels exhibit improved filtration performance.

This is a divisional of application Ser. No. 08/941,945 filed Oct. 1,1997.

The present invention pertains to electret fibers and electret filtermedia made of fibers such as melt-blown polymer microfibers and methodsof making electret fibers and filters. More specifically, the inventionpertains to electret fibers containing a low level of extractablehydrocarben material. The level of extractable hydrocarbon material is afunction of the polymer selected and the processing conditions employedto make the fibers and filters.

BACKGROUND

Electret articles comprise a dielectric material exhibiting a persistentor quasi-permanent electrical charge. See G. M. Sessler, Electrets,Springer Verlag, N.Y. (1987). The articles are commonly used in the formof fibrous filtering webs for applications, and processes for makingelectret nonwoven fibrous filter webs are very well known. For example,nonwoven webs can be made from polymers using melt-blowing techniques,such as those described in Van Wente, “Superfine Thermoplastic Fibers,”Ind. Eng. Chem., vol. 48, pp. 1342-46, (1956), and an electric chargecan be imparted in the web using various techniques. (See U.S. Pat. No.4,215,682; 4,588,537; 5,411,576 and 5,472,481; 5,645,627; 5,496,507; andWO 97/07272).

Because of the importance of air filtration and the desirable propertiesthat electret filter webs have shown in filter applications,considerable efforts have been devoted to improving the performance offibrous electret filters. The above-cited patents reflect some of thework that has been reported to improve electret filter performance, andwhat follows is a brief summary of these contributions.

Kubik and Davis in U.S. Pat. No. 4,215,682 imparted an electric chargein melt-blown fibers by bombarding the fibers with electrically chargedparticles as the fibers issued from a die orifice.

Klaase et al. in U.S. Pat. No. 4,588,537 injected charge into anelectret filter using a corona treatment.

Jones et al. in U.S. Pat. Nos. 5,411,576 and 5,472,481, discloseelectret filters that are made by extruding a blend of polymer with amelt-processable fluorochemical in a microfibrous web. The resulting webis annealed and corona treated.

Lifshutz et al. in U.S. Pat. No. 5,645,627 (WO 96/26783) makes electretfilters by extruding a blend of polymer with a fatty acid amide or afluorochemical oxazolidinone or a mixture of these, in a microfibrousweb, followed by annealing and corona treating the resulting web.

Angadjivand et al. in U.S. Pat. No. 5,496,507 indicate that impingingwater droplets onto a nonwoven microfiber web imparts a charge to theweb.

Rousseau et al. in WO 97/07272 disclose electret filters that are madeby extruding blends of a polymer with a fluorochemical or organictriazine compound into a microfiber web, followed by impinging waterdroplets onto the web. This publication indicates that use of theseadditives results in improved charge when the web has been impinged bywater droplets.

Although the above documents disclose a variety of methods for improvingelectret filter performance, the previous efforts have nonetheless leftroom for further contributions and the invention described below is yetanother discovery directed toward the ongoing effort of establishingbetter electret fibers and filters.

SUMMARY OF THE INVENTION

The present invention-provides electret fibers and filters comprised ofa polymeric material and a fluorochemical additive. The fibers have lessthan about 3 weight percent of extractable hydrocarbon material based onthe weight of the fibers. The level of extractable hydrocarbon materialis measured by extracting the fibers in CHCl₃ for 10 minutes at roomtemperature and measuring the amount of material that has dissolved outfrom the fibers.

The inventive fibers having less than about 3 weight percent ofextractable hydrocarbon material can be made by a process of blending apolymer with a fluorochemical additive compound, extruding the blend ata temperature maintained below 290° C. to form extruded fibers, andannealing and charging the extruded fibers.

The present inventors have discovered that there is a correlationbetween the level of extractable hydrocarbon material found in anelectret filter and the filter's loading performance. Surprisingly, theinventors discovered that the lower the level of extractablehydrocarbons in the extruded web, the better the loading performance ofthe web (loading performance involves a filter's ability to remove anoily aerosol from a gas stream and is described in detail in theExamples section). An electret filter web's performance thus can bepredicted by measuring the level of extractable hydrocarbons.

The extractable hydrocarbon level in the extruded web is a function ofpolymer type and processing conditions used to make the web. Selectingthe correct polymer type can be important to achieve a low level ofextractable hydrocarbon material in the filter web's fibers. Harshprocessing conditions such as peroxide use and high extrusiontemperatures should be avoided because they can increase extractablehydrocarbons and cause a corresponding decrease in loading performance.Thus, controlling the foregoing parameters can result in a filter webexhibiting improved loading performance.

The electret fibers and filters of the present invention find multipleuses including, but not limited to, use in: respirators such as facemasks, home and industrial air-conditioners, air cleaners, vacuumcleaners, medical and other air line filters, and air conditioningsystems in vehicles and electronic equipment such as computers and diskdrives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plot of aerosol loading performance as measured by theminimum at challenge—that is, the mass of dioctylphthalate (DOP)incident on a filter web at the point where the DOP Percent Penetrationreaches a minimum value, hereinafter “Min@Chl”—of three filter websamples containing different weight percents of extractable hydrocarbonmaterial. As explained in detail in the Examples section, this data wasobtained by exposing the filter webs to a DOP liquid aerosol in aninstrument that measures the concentration of DOP liquid aerosolupstream and downstream to the filter. The Percent Penetration iscalculated by dividing the aerosol concentration downstream by theconcentration upstream and multiplying by 100.

FIG. 2 shows a plot of aerosol loading performance as measured byMin@Chl for 17 samples containing different weight percents ofextractable hydrocarbon material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The polymeric material used in the invention is selected such that,after being extruded and converted into an electret fiber under selectedconditions, the electret fibers have a low level of extractablehydrocarbon. The polymeric material can be a nonconductive thermoplasticresin—that is, a resin having a resistivity greater than 10¹⁴ ohm·cm.Preferred polymeric materials have a resistivity greater than 10¹⁶ohm·cm. The polymeric material should have the capability of possessinga persistent or long-lived trapped charge. It is contemplated that thepolymers are selected, as described below, by routine experimentation todetermine those exhibiting low levels of extractable hydrocarbons. Thepolymeric material may include, but is not necessarily limited to,polyolefins such as polyethylene and polypropylene,poly-4-methyl-1-pentene, polyvinylchloride, polystyrene, polycarbonateand polyester and combinations of these polymers.

Polypropylene is a preferred polymer because of polypropylene's highresistivity, its ability to form melt-blown fibers and its satisfactorycharge stability, hydrophobicity, and resistance to humidity. Examplesof preferred polypropylenes include Escorene PP-3505G (100% isotacticpolypropylene, density 0.91 g/cc, melt index 400 g/10 min, m.p. 160°C.). available from Exxon Corporation; and Fina 3860 (density 0.905g/cc, melt index 100 g/10 min, m.p. 165° C.), available from Fina Oiland Chemical Company.

Fluorochemical additives useful in the present invention can providewater repellency to fibers. The fluorochemical additives are desirablymelt processable—that is, they suffer substantially no degradation underthe melt processing conditions that are used to form the fibers. Thefluorochemical additive should be solid at 25° C. and preferably has amelting point of at least about 70° C., more preferably at least about100° C. The fluorochemical additive preferably exhibits no phasetransitions in the range of commonly encountered temperatures, i.e.,about 0 to 80° C. as such changes in molecular freedom may adverselyaffect charge stability. The fluorochemical additive preferably has amolecular weight of about 500 to 2500, more preferably about 800 to1500. The fluorochemical additive preferably is substantially free frommobile polar and/or ionic species, contaminants and impurities thatcould increase the electrical conductivity or otherwise interfere withthe fiber's ability to accept and hold electrostatic charges.

Preferred fluorochemical additives include, for example, fluorochemicaloxazolidinones as described in U.S. Pat. No. 5,025,052 to Crater et al.,fluorochemical piperazines as described in Katritzky, Alan R. et al.,“Design and Synthesis of Novel Fluorinated Surfactants for HydrocarbonSubphases,” Langmuir, vol. 4, pp. 732-735, (1988), and perfluorinatedalkanes preferably having about 10 to 50 carbon atoms, more preferablyabout 15 to 30 carbon atoms. The fluorochemical additive preferably ispresent in amounts of up to 10 weight percent, more preferably about 0.4to 5 weight percent, most preferably about 0.5 to 2 weight percent.Especially preferred fluorochemical additives are those described inJones et al., U.S. Pat. No. 5,411,576.

In a method of producing the inventive fibers, the polymer andfluorochemical additive are blended before extruding. The blending stepcan be conducted by blending the solids before adding them to theextruder but are preferably sparately melted and blended together asliquids. More preferably, the fluorochemical additive is blended in afirst extruder in an amount of about 10 to about 20 weight percent whenusing propropylene to form the fibers. This relatively highfluorochemical-content molten blend is fed into a second extrutder thatcontains molten polypropylene without a fluorochemical. The resultingblend is then extruded into fibers.

The extruders preferably are twin screw extruders. Temperature duringextrusion should be controlled to provide desired melt rheology andavoid flourochemical thermal degradation. Preferably the temperatureduring extrusion is maintained below 290° C. Extrusion temperaturesabove 290° C. can cause the level of extractables to rise above 3percent and cause a corresponding decrease in loading performance. Morepreferably the time at high temperature is minimized by introducing theblend of the polymer melt with 10-20 percent fluorochemical at about210° C. at a point near the end of the second extruder.

The extrudate can be made into fibers using known or later developedfiber-forming methods, including spinning or melt-blowing. Themelt-blowing technique has been originally described by Van Wente,“Superfine Thermoplastic Fibers” Ind. Eng. Chem., vol. 48, pp. 1342-46,(1956), and uses gas streams to draw fibers out from an extruderorifice. The melt-blowing method is desirable for its ability toefficiently produce fine fibers that can easily be collected as anonwoven web—that is handleable by itself. The fibers obtained bymelt-blowing are known as blown microfibers or BMF.

The extruded fibers may be collected in the form of a nonwoven web byknown methods including, but not limited to, collecting BMF as describedin Van Wente, “Superfine Thermoplastic Fibers.”

The fibers or the nonwoven web can be annealed in order to increase theelectrostatic charge stability in the final product, particularlystability to liquid aerosols. Preferably, the fluorochemical is asubstance that presents low energy surfaces and the annealing step isconducted at a sufficient temperature and for a sufficient time to causethe fluorochemical to bloom to the interfaces (e.g., the fiber surfaceor boundaries between crystalline and amorphous phases within the fiber)of the fibers. Generally, higher annealing temperatures reduce annealingtimes. Preferably, annealing is is conducted at about 130 to 155° C. forabout 2 to 20 minutes, more preferably at about 140 to 150° C. for about2 to 10 minutes, and most preferably at about 150° C. for a period ofabout 4.5 minutes. Annealing temperatures above about 155° C. aregenerally undesirable because the fibers or web can be damaged.

The fibers are then electrostatically charged. Examples of electrostaticcharging method useful in the present invention are described in U.S.Pat. Nos. Re. 30,782 to van Turnhout, Re. 31,285 to van Turnhout,4,275,718 to Wadsworth et al., 4,588,537 to Klaase et al., and 4,592,815to Nakao. Fibers can be hydrostatically charged, and cut fibers can betribocharged by rubbing or by shaking with dissimilar fibers. See, forexample, U.S. Pat. No. 4,798,850. Preferably, the web is subjected to acorona discharge or pulsed high voltage as described in the patentscited in the Background section.

The electret fibers and filters of the present invention exhibit lowextractable hydrocarbon levels. Preferably, the web has about 1.0 to 3.0weight percent extractable hydrocarbon material, more preferably about1.0. to 2.5 weight percent, and most preferably about 1.0 to 1.5 weightpercent. Hydrocarbons are compounds made up of only carbon and hydrogenand in the present invention can include small amounts of oxygen such asmight be introduced by a peroxide. Extractable hydrocarbon material iscalculated as follows.

A sample of a collection of fibers, such as a nonwoven web, is preparedfor extractable hydrocarbon material analysis by weighing 50 milligrams(mg) of the web into a four dram (16 milliliters (ml)) vial, adding 10ml of chloroform to the vial and shaking the sealed vial for 10 minuteson an automatic shaker (such as a wrist action shaker) at roomtemperature. The amount of extracted hydrocarbon is quantified by anappropriate technique such as high performance liquid chromatography(HPLC). The extracted fluorochemicals elute at different times than theextracted hydrocarbons and can be quantified separately. The weightpercent extractable hydrocarbon material does not include the weight ofextractable fluorochemicals. To obtain weight percent extractablehydrocarbon material, the weight of the extracted hydrocarbon materialis divided by the weight of the fibers (50 mg) and multiplied by 100.

Fibers for fibrous electret filters of the invention typically have aneffective fiber diameter of from about 5 to 30 micrometers, andpreferably from about 6 to 10 micrometers as calculated according to themethod of Davies, “The Separation of Airborne Dust and Particulates,”Institution of Mechanical Engineers, Proceedings 1B, (1952).

Electret fibers resulting from the processes described above may beformed into an electret filter. An electret filter can take the form ofa nonwoven web containing at least some electret fibers, or electretfibers combined with a supporting structure. In either case, theelectret article can be combined with some nonelectret material. Forexample, the supporting structure can be nonelectret fibers orsupporting nonelectret, nonwoven webs. The electret filter is preferablya nonwoven electret web containing electrically-charged, melt-blownmicrofibers.

Electret filter webs may also include staple fibers that generallyprovide a loftier, less dense web. Methods of incorporating staplefibers in the nonwoven web can be carried out as described U.S. Pat. No.4,118,531 to Hauser. If staple fibers are used, the web preferablycontains less than 90 weight percent staple fibers, more preferably lessthan 70 weight percent. For reasons of simplicity and optimizingperformance, the electret web may in some instances consist essentiallyof melt-blown fibers and does not contain staple fibers.

The electret filter may further contain sorbent particulates such asalumina or activated carbon. The particulates may be added to the filterto assist in removing gaseous contaminants from an airstream that passesthrough the filter. Such particulate loaded webs are described, forexample, in U.S. Pat. Nos. 3,971,373 to Braun, 4,100,324 to Anderson and4,429,001 to Kolpin et al. If particulate material is added, the webpreferably contains less than 80 volume percent particulate material,more preferably less than 60 volume percent. In embodiments where theelectret filter does not need to remove gaseous contaminants, the filtermay contain only melt-blown fibers.

The electret filter should be substantially free of components such asantistatic agents that could increase the electrical conductivity orotherwise interfere with the fibers' ability to accept and holdelectrostatic charge. Additionally, the electret filter should not besubjected to treatments such as exposure to gamma rays, UV irradiation,pyrolysis, oxidation, etc., that might increase electrical conductivity.Thus, in a preferred embodiment the electret filter is not exposed togamma irradiation or other ionizing radiation.

The electret filters typically have a basis weight of about 10 to 500grams per meter squared (g/m²), more preferably about 10 to 100 g/m².Filters that are too dense may be difficult to charge while those thatare too light or too thin may be fragile or have insufficient filteringability. For many applications the electret filters are about 0.25 to 20millimeters (min) thick, and commonly about 0.5 to 2 mm thick. Electretfilters of these sizes may be particularly useful in a respirator.

In respirators, the fibrous electret webs may be specially shaped orhoused, for example, in the form of molded or folded half-face masks,replaceable cartridges or canisters, or prefilters. Respirators may alsohave additional features such as additional layers, valves, molded facepieces, etc. Respirator examples that can incorporate the improvedelectret filters of the invention include those described in U.S. Pat.Nos. 4,536,440, 4,827,924, 5,325,892, 4,807,619 4,886,058 and U.S.patent application Ser. No. 08/079,234.

The following examples show that webs having a relatively lowconcentration of extractable hydrocarbon material exhibit higher loadingperformance than webs having relatively high concentrations ofextractable hydrocarbon material. The level of extractable hydrocarbonmaterial found in a web is a function of the polymer and the processconditions used to make the web. Because the level of extractablehydrocarbon in the web influences the web's loading performance, thisparameter can be controlled by polymer selection and the processingconditions employed in making the web.

EXAMPLES

General Sample Preparation

Extrusion of Webs

Polypropylene BMF webs containing a fluorochemical melt additive wereextruded using a two extruder process. The fluorochemical melt additivewas fed into the throat of a twin screw extruder along withpolypropylene resin to produce a melt stream that contained about 11weight percent fluorochemical. The bulk of the polypropylene resin wasadded to the throat of a second twin screw extruder. In some cases, aperoxide was also metered in to reduce viscosity. The output of thefluorochemical-containing extruder was pumped into thepolypropylene-containing extruder at a rate such as to make the totaloutput about 1.1 percent by weight fluorochemical melt additive.

For consistency and control of variables, the additive used in everysample was additive A of U.S. Pat. No. 5,411,576 having the formula

The melt stream temperature containing the fluorochemical melt additivewas maintained below 290° C. at all points. The final melt temperaturewas 288° C. The extrusion conditions for the main polymer melt stream,upstream to where the fluorochemical was introduced, were adjusted toproduce the desired properties in the extruded webs. When low melt indexresins were used or extrusion rates were increased to more than 50pounds per hour, (2,5-dimethyl-2,5-ditert-butylperoxy)hexane was co-fedinto the extruder to control the polymer's melt rheology and thephysical parameters of the melt blown web. The web itself was producedin a conventional manner similar to that described in Van Wente, et. al.except that a drilled orifice die was used. Webs were made to one of theweb specifications (Web Spec) below unless otherwise indicated.

Basis Weight Pressure Drop Thickness Web Spec (g/m²) (mmH₂O) (mm) 1 60.74.4 1.27 2 71.4 7.0 1.35 3 58.2 4.1 1.32 4 59.0 5.8 1.21 5 70.9 4.1 1.386 85.5 7.9 1.59

Annealing

The extruded webs were further treated by passing them through an ovenat an average temperature of about 150° C. at a rate such that the dwelltime in the oven was about 4.5 minutes. This annealing process causedadditional polymer crystallization and caused the fluorochemical meltadditive to bloom the fiber surfaces.

Charging

The webs were corona charged using a high voltage electric fieldprovided between 30 linear cross-web corona sources and a groundelectrode having a corona current of 2.6·10⁻³ milliamps/cm of coronasource length and a residence time of about 15 seconds.

DOP Loading Test

The dioctylphthalate (DOP) loading measurement is performed bymonitoring the penetration of DOP aerosol through a sample duringprolonged exposure to a controlled DOP aerosol. The measurements aremade with a TSI Incorporated Automated Filter tester (AFT) model #8110or #8130 adapted for DOP aerosol.

The DOP % Penetration is defined as follows:

DOP % Penetration=100(DOP Conc. Downstream/DOP Conc. Upstream), wherethe concentrations upstream and downstream are mieasuired by lightscattering chambers. The concentrations are measured and the DOP %Penetration is calculated automatically by the AFT. The DOP aerosolgenerated by the 8110 and 8130 AFT instruments is nomninally amonodisperse 0.3 micrometers mass median diameter and has an incident(upstream) concentration of 100 milligrams per cubic meter as measuredby a standard filter. The samples tested were all tested with a flowrate through the filter web sample of 85 liters per minute (1/min). Allsamples reported here were tested with the aerosol ionizer turned off.The samples tested were discs 5.25 inches (13.34 cm) in diameter with anarea 4.5 inches (11.43 cm.) In diameter exposed. The face velocity was13.8 cm/sec.

The sample discs were weighed and then two of the discs were stackeddirectly on top of each other and mounted in the AFT. Each test wasbegun and continued until there was a clear trend for increasing DOP %Penetration with continued DOP aerosol exposure or at least until anexposure to 200 milligrams of DOP. The DOP % Penetration andcorresponding Pressure Drop data were transmitted to an attachedcomputer where they were stored. After the termination of the DOPloading test, the loaded samples were weighed again to monitor the DOPamount collected on the fibrous web samples. This was used as across-check of the DOP exposure extrapolated from the measured DOPconcentration incident on the fibrous web and the measured aerosol flowrate through the web.

The resulting loading data was imported into a spread sheet to calculatethe Minimum @ Challenge (Min@Chl). The Min@Chl is defined to be thetotal DOP challenge or mass of DOP which has been incident on the filterweb at the point where the DOP % Penetration reaches its minimumpenetration. This Min@Chl is used to characterize web performanceagainst DOP loading, the higher the Min@Chl the better the loadingperformance.

Determination of Extractable Hydrocarbons

Web samples were prepared for analysis by weighing 50 milligrams (mg) ofweb into a 4 dram vial, adding 10 ml of chloroform to the vial and thensealing the vial with a Teflon lined cap. The vial was shaken for 10minutes on a wrist action shaker, and the extract was analyzed by HPLCunder the following chromatographic conditions:

Column: Alltech CN 5 μm, ×150 mm

Solvent A: Hexane

Solvent B: 5% Methanol in methylene chloride

Gradient: 10% B to 100% B in 20 minutes

Flow Rate: 0.25 ml/min

Injector: 2 μL

Detector: Evaporative Light Scattering Detector, gain=8

A polypropylene standard (weight average molecular weight of 830, numberaverage molecular weight of 740) from American Polymer Standard Corp.was used to make up a series of polypropylene standard solutions inchloroform in the concentration range of 1000 to 60 micrograms permilliliter (μg/ml). These standard solutions were analyzed and acalibration curve was calculated from the linear regression analyses ofthe log concentration versus log chromatographic area. This calibrationcurve was then used to determine the level of hydrocarbon material (inthis case, polypropylene) extracted from the web samples.

EXAMPLES 1-3: FIG. 1

BMF web was prepared from various polypropylene resins and thefluorochemical melt-additive Additive A in U.S. Pat. No. 5,411,576 at arate of 50 lb/hr and an extrusion temperature of 288° C. The webs weremade to have the parameters in Web Specification 1. Peroxide was addedto the Fina 3860 resin to control melt rheology. After annealing andcharging the web as described above, DOP load testing was performed onat least sixteen 5.25 inch samples taken from across and down the webfor each of Examples 1-3. The weight percent extractable hydrocarbonmaterial was determined for samples from the same webs using the HPLCtechnique described above. The measurements of weight percentextractable hydrocarbon material were more precise (within 5% precision)than the Min@Chl measurements. Therefore only about 2 extractionmeasurements were conducted for each example while at least 16 sampleswere tested to obtain the Min@Chl value for each of Examples 1-3. Theloading performance and extraction data are set forth in Table 1 andplotted in FIG. 1.

TABLE 1 Wt. % Extractable Hydrocarbon Example Number Resin Type MaterialMin@Chl 1 Exxon Escorene 1.0 270 3505G 400 melt index 2 Fina 3860 2.4163 100 melt index 3 Fina HMF 3860 4.2  34 (EOD 94-18) 400 melt index

As illustrated by the data in Table 1 and FIG. 1, liquid aerosol loadingperformance, as measured by Min@Chl, correlates inversely withincreasing weight percent extractable hydrocarbon in the webs. The lowerthe level of extractable hydrocarbon the greater the value of theMin@Chl.

EXAMPLES 4-17

Additional webs were prepared as described above and according to theconditions and web specifications set forth in Table 2.

TABLE 2 Polymeric Extrusion Example No. Material Rate, lb/hr Web SpecPeroxide 4 Exxon 50 1 No 3505G 5 Fina 3860 50 1 Yes 6 Fina HMF 50 1 No3860 7 Exxon 50 2 No 3505G 8 Fina HMF 50 2 No 3860 9 Exxon 75 2 Yes3505G 10 Fina 3860 75 2 Yes 11 Exxon 100 2 Yes 3505G 12 Fina HMF 100 2Yes 3860 13 Exxon 50 2 No 3505G 14 Exxon 50 3 No 3505G 15 Exxon 50 4 No3505G 16 Exxon 50 5 No 3505G 17 Exxon 50 6 No 3505G

Examples 4-17 were annealed and corona charged and load tested asdescribed above for Examples 1-3 except that only three samples fromeach web were tested and averaged to obtain the Min@Chl. The weightpercent of extractable hydrocarbon was also determined as describedabove for each example including remeasuring the webs Examples 1-3 toinsure precise extraction values among Examples 1-17 and to compensatefor any changes in column conditions that may have occurred between theinitial testing of Examples 1-3 and the testing of Examples 1-17. Theweight percent extractable hydrocarbon material and Min@Chl for Examples1-17 are presented in Table 3 and plotted in FIG. 2.

TABLE 3 Wt % Extractable Example No. Hydrocarbon Min@Chl 1 1.1 270 2 2.1163 3 3.4 34 4 1.1 250 5 2.3 124 6 3.5 57 7 1.1 220 8 2.5 87 9 1.5 78 102.4 62 11 1.3 83 12 4.0 23 13 1.1 71 14 1.4 260 15 1.2 112 16 1.1 188 171.2 183

The data in Table 3 and FIG. 2 show the general trend that increasinglevels of extractable hydrocarbons correlate generally with decreasingloading performance (i.e., decreasing Min@Chl). Values for several ofthe examples deviate from the linear relationship illustrated in FIG. 1.This deviation is believed to be due, at least in part, to largerexperimental error of the Min@Chl data resulting from measuring fewersamples for Examples 4-17 (Min@Chl values for Examples 1-3 were theresult of averaging measurements from 16 samples while Min@Chl valuesfor Examples 4-17 were the result of averaging the measurements of only3 samples).

As shown in Table 3 and FIG. 2, the filter webs with the highest weight% extractable hydrocarbons, Examples 3, 6 and 12, having 3.4%, 3.5% and4.0% extractable hydrocarbons respectively, exhibited the poorestloading performance, having Min@Chl values of 34, 57 and 23respectively. Thus, the data shows that filter webs with more than about3.0 weight percent extractable hydrocarbon material have undesirableloading performance.

Examples 1, 4, 7, 14, 16 and 17 having weight % extractable hydrocarbonmaterial of 1.1, 1.1, 1.1, 1.4, 1.1, and 1.2%, respectively,demonstrated the best loading performance as shown by their respectiveMin@Chl values of 270, 250, 220, 260, 188 and 183. Thus, the data inTable 3 and FIG. 3 show that the best loading performance is obtainedfor webs having about 1.0 to 1.5 weight percent extractablehydrocarbons.

The data also illustrate the importance of selecting the proper polymerto prepare the electret fibers and nonwoven webs. All the examples thatused Fina HMF 3860 polypropylene, Examples 3, 6 and 12, had the highestlevels of extractable hydrocarbon material and exhibited the poorestloading performance as evidenced by their Min@Chl values.

The references, including U.S. patents, that are mentioned herein areincorporated by reference as if reproduced in full below. In describingpreferred embodiments of the present invention specific terminology hasbeen employed forthe sake of clarity. The invention, however, is notintended to be limited to the specific terminology so selected, and itis to be understood that each specific element includes all technicalequivalents that operate similarly to accomplish a similar purpose. Itis therefore to be understood that, within the scope of the appendedclaims and their equivalents, the invention may be practiced otherwisethan as specifically described.

What is claimed is:
 1. A method of predicting the performance of anelectret filter web comprising the step of measuring the level ofextractable hydrocarbons in the web.
 2. A filter that comprises theelectret filter web of claim
 1. 3. A respirator or filtering face maskthat employs the filter of claim
 2. 4. The method of claim 3, furthercomprising replacing the electret filter in a respirator.
 5. The methodof claim 1, wherein the electret filter comprises a nonwoven web ofmicrofibers.
 6. The method of claim 5, wherein the microfibers aremelt-blown microfibers that contain a nonconductive thermoplastic resin.7. The method of claim 6, wherein the thermoplastic resin has aresistivity greater than 10¹⁶ ohm·cm.
 8. The method of claim 6, whereinthe electret filter exhibits persistent trapped charge.
 9. The method ofclaim 8, wherein the thermoplastic resin includes a polymer that ispolyethylene, polypropylene, poly-4-methyl-1-pentene, polyvinylchloride,polystyrene, polycarbonate, polyester, or a combination thereof.
 10. Themethod of claim 9, wherein the polymer is polypropylene, orpoly-4-methyl-1-pentene.
 11. The method of claim 10, wherein the fibersfurther comprise a fluorochemical additive.
 12. The method of claim 11,wherein the fibers have an effective fiber diameter of about 5 to 30micrometers.
 13. The method of claim 12, wherein the electret filter hasa basis weight of 10 to 500 g/m² and has a thickness of about 0.25 to 20mm.
 14. The method of claim 12, wherein the electret filter has a basisweight of 10 to 100 g/m² and has a thickness of about 0.5 to 2 mm. 15.The method of claim 1, wherein the electret filter web comprisespolymeric fibers.
 16. The method of claim 15, wherein the polymericfibers comprise a nonconductive thermoplastic resin.
 17. The method ofclaim 16, wherein the fibers have a resistivity greater than 10¹⁴ohm-cm.
 18. The method of claim 17, wherein the fibers are microfibers.19. The method of claim 18, wherein the microfibers containpolypropylene and fluorine.
 20. The method of claim 16, wherein thefibers have a resistivity greater than 10¹⁶ ohm-cm.